It is known to arrange auxiliary brakes in a vehicle as a complement to the service brakes of the vehicle. Auxiliary brakes are primarily used in heavy-duty vehicles with the principal aim of saving the service brakes of the vehicle, especially when driving on long downhill slopes where there is a desire to brake to maintain fairly constant speed. The use of the auxiliary brakes allows the service brakes to be kept fresh, so that they can provide maximum braking force when the vehicle really does need to slow down quickly. The service brakes have a much more powerful braking action than the auxiliary brakes, partially owing to the fact that the service brakes are normally arranged on all the wheels of the vehicle. The auxiliary brakes normally act only on the driving wheels.
It is further known to differentiate between so-called primary and secondary auxiliary brakes in a vehicle. Primary and secondary alludes to the positioning of the auxiliary brake before or after the main gearbox of the vehicle. Examples of primary auxiliary brakes are ISG (Integrated Starter & Generator) and retarders. A retarder is usually of the hydrodynamic retarder or the electromagnetic retarder type. These are disposed between the engine and the main gearbox. A primary auxiliary brake can also be constituted by various types of engine brakes, for example compression brake, exhaust brake or the internal friction of the engine. The braking energy in a compression brake and exhaust brake is converted mainly into heat, which, in large part, is dissipated via the engine cooling system, though it should be noted that a substantial part (about 40% of the braking energy) accompanies the vehicle exhaust gases out through the exhaust system. The brake power which can be delivered by a primary auxiliary brake is dependent on the engine speed, and for this reason it is advantageous to maintain a relatively high engine speed whenever a primary auxiliary brake is used.
The internal friction of the engine can be adjusted by injecting a certain quantity of fuel into the engine, for example, so that the output torque from the engine becomes zero when no brake power is wanted. Another option for avoiding internal friction of the engine is to disengage the engine from the rest of the drive line by means of a clutch disposed between the engine and the gearbox. In the present context, drive line is meant to include the vehicle engine, as well as transmission components coupled to the engine, and continuing up to the drive wheels. Other controllable units which are coupled to the engine also contribute to the braking force from the engine; i.e., are added to the brake torque from the internal friction of the engine. Examples of such units are the cooling fan of the engine, the air conditioning unit of the vehicle, air compressors, generators and other accessory units coupled to the engine.
In the present disclosure, the term “friction torque of the engine” is used to denote brake torque that is obtained from the internal friction of the engine with connected units, but without any other auxiliary brakes being connected. The term “engine brakes” embraces compression brake, exhaust brake and the friction torque of the engine.
A secondary auxiliary brake, which is disposed somewhere after the main gearbox of the vehicle, is usually constituted by a retarder of the hydrodynamic or electromagnetic type. The brake power which can be delivered by a secondary auxiliary brake is dependent on the speed of the vehicle since the auxiliary brake is mounted on the output shaft of the gearbox and is therefore proportional to the rotation speed of the drive wheels.
When a vehicle is driven on a downhill slope, the brake power of the auxiliary brake may not prove sufficient, but rather the driver may instead need to support this with the service brake in order to maintain a low and regular vehicle speed. On certain occasions, the brake power from an auxiliary brake can be cut-off without the driver expecting it, which can be disquieting for the driver.
Such a situation occurs when the vehicle is being driven on a steep downhill slope at a low speed and the vehicle is equipped with a semiautomatic gearbox; i.e., an automatically shifted stage-geared gearbox. These gearboxes are often non-synchronized and the vehicle has no manual clutch pedal for disconnecting the clutch in the transmission. When the driver brakes the vehicle to reduce the speed, the engine speed will also decrease. When the engine speed approaches the idling speed of the engine, the system will disengage the gearbox; i.e., the power transmission between the engine and the transmission is broken. This disengagement is realized to prevent the vehicle from driving the vehicle forward with the aid of the idle regulator and prevent the engine from being throttled down. This means, at the same time, that the brake torque deriving from the internal friction of the engine, and any primary auxiliary brakes, disappears. The result is a sudden reduction in braking torque which can cause the vehicle to start to accelerate. This can be disquieting for the driver because he senses a reduced brake power even though he is stepping on the service brake.
There is therefore a need to be able to distribute the brake torque between service brakes and engine brakes in a vehicle in a way which compensates for the loss of brake power from the engine brakes. This is the main aim of the invention which is described below.